The invention relates to fluid filters, and more particularly to a liquid filter with an improved capability to be cleaned of clogging filtrate material during a washing or "blow-down" cycle.
Modern irrigation methods often require rather accurate water delivery at controlled predetermined rates to crops and even to individual plants. This has led to the development of irrigation systems having water emitters with numerous small passages which have been subject to blockage due to the presence of silt and other suspended materials in the irrigation water. To remove such particles from irrigation water prior to its flow through these small conduits, various types of filters were developed and adapted for use in irrigation systems with varying degrees of success. The filters were subjected to very large flow rates often in the hundreds of gallons per minute, and were also required to handle water from a number of sources, each with its own quantity and type of particles, including organic suspended matter. In order to reduce filter down time for cleaning, backflushing and forward flushing devices for the filter systems were introduced. Some of the flushing systems have been incorporated in cylindrical type filters wherein a filter mesh sleeve separates filtrate by radial outward flow. In such filters the mesh is cleaned by axial flow through the sleeve substantially without passing through the mesh, during a "blow-down" cycle. The heavily filtrate-laden wash water is discharged out the end of the filter assembly as waste. Usually mesh screen is retained in place in the elongated filter assembly by a more rigid sleeve immediately surrounding it.
Cylindrical filter sleeves of synthetic fiber mesh have been fairly effective in such self-cleaning filters particularly with loose sand or other fairly coarse filtrate material. However, these meshes do not clean well where materials with a tendency to cake are present such as algaes and other organic substances. When these caking filtrate materials have been deposited on the filtering surface, they have tended to hold the filtering sleeve fast against its rigid retaining screen and the passing blow-down flow has been of little effect in removing the cake from the mesh. The principal problem has been the lack of any means of disengaging the caked mesh from the rigid back-up screen so that the filtrate cake can be shaken off. This break-away must occur consistently during blow-down for there to be consistently effective cleaning.
Self-cleaning filters developed heretofore have widely varied in their cleaning effectiveness depending upon various factors such as the blow-down flow rate and the character of the materials suspended in the water. Consistency of cleaning effectiveness has thus not been achieved, greatly diminishing the filtration capability of such filters as well as causing other problems.
One such problem has been the risk of rupture of a clogged filtering sleeve. When a filter mesh is fully blocked and full line pressure is allowed to build up in the interior of the sleeve prior to cleaning or following an ineffective cleaning, a very large pressure drop occurs across the filtering mesh. If a rigid retaining sleeve is provided surrounding the mesh sleeve, the areas of large pressure drop occur at the locations of the openings in the retaining sleeve. Thus, whether or not such a restraining sleeve is provided, the mesh has tended to rupture with obvious consequences, when blow-down mesh cleaning has not been effective.
In filter assemblies equipped with these relatively rigid, usually perforated retaining sleeves around the filter mesh, the openings or perforations have been excessively small and/or few, often comprising considerably less than one-third of the total area of the retaining sleeve and leaving a correspondingly large closed area. Pressure differentials on this large closed area of the retaining sleeve when the mesh is clogged or partially clogged have consequently built up to high levels more easily. Rapid pressure fluctuations often occur which subject the sleeve to repeated localized flexing leading to fracture of the sleeve. Such a failure of course further subjects the mesh to rupture.